Porcine circovirus (PCV) was originally identified as a contaminant of porcine kidney cell cultures (PK15 ATCC CCL-33). The PCV virion has been characterized as being an icosahedral, non-enveloped virus with a single-stranded circular DNA of about 1.76 kb. PCV was classified in the genus Circovirus of the Circoviridae family, which consists of other animal circoviruses such as psittacine beak-feather disease virus, goose circovirus, canary circovirus, and pigeon circovirus. Two genotypes of PCV have been recognized. The PK15 cell-derived PCV has been considered to be nonpathogenic to pigs, and is designated PCV type 1 (PCV1). On the other hand, PCV type 2 (PCV2) has been accepted as the major infectious agent involved in several pig diseases. PCV2 associated diseases cause significant economic losses to swine producers worldwide. PCV2 associated diseases are described in WO2007/076520 and include, for example, Postweaning Multisystemic Wasting Syndrome (PMWS), Porcine Dermatitis and Nephropathy Syndrome (PDNS), Porcine Respiratory Disease Complex (PRDC), reproductive disorders, granulomatous enteris, exsudative epidermitis, necrotizing lymphadenitis, and congenital tremors. Occurrences of PCV2 subtype A (PCV2A) and PCV2 subtype B (PCV2B) have been reported particularly in 2000 in West Europe and in Central Europe in 2003. More recently similar changes have been reported in 2008 in wild boars.
Currently developed PCV2 vaccines, such as Circovac® (Merial), Ingelvac®, CircoFLEX (Boehringer Ingelheim Vetmedica), or Suvaxyn®, are either inactivated PCV2 vaccines or Sub-Unit vaccines. Regarding inactivated PCV2 vaccines, current PCV2 strains subtype A or B present several weaknesses. Particularly, PCV2 viruses can only be produced at low titers, generally less than 10<5> TCID50 viral particles per ml. Also, these viruses cannot be maintained in tissue cultures and permanently infected cell lines. Regarding PCV2A Sub-Unit vaccines, they typically use a purified, recombinant PCV2A capsid protein produced by expression of the ORF2 gene of PCV2A in a baculovirus system. In this regard, the protein encoded by ORF2 of PCV2 isolates Imp1011 has been reported in EP1741785. A protein encoded by ORF2 of PCV2 isolate PCV2Rm has been reported in WO2010/061000. The protein encoded by ORF2 of PCV2 isolate 412 has been reported in EP1816200. Another protein encoded by an ORF2 of a further PCV2 isolate has been reported in EP1036180 or EP2225367.
Expression efficiency and immunogenicity of these natural capsid proteins are, however, not optimal and do not always provide the required level of immune protection in vaccinated animals.
Accordingly, there is a need for alternative vaccine compositions to improve the efficacy of treatment or prevention of PCV2 infections in mammals, particularly in pigs.